Re: [RFC] Request API and V4L2 capabilities

Hans Verkuil
2018-08-06T10:40:24Z

On 08/06/2018 10:16 AM, Paul Kocialkowski wrote:
> Hi Hans and all,
>
> On Sat, 2018-08-04 at 15:50 +0200, Hans Verkuil wrote:
>> Hi all,
>>
>> While the Request API patch series addresses all the core API issues, there
>> are some high-level considerations as well:
>>
>> 1) How can the application tell that the Request API is supported and for
>> which buffer types (capture/output) and pixel formats?
>>
>> 2) How can the application tell if the Request API is required as opposed to being
>> optional?
>>
>> 3) Some controls may be required in each request, how to let userspace know this?
>> Is it even necessary to inform userspace?
>>
>> 4) (For bonus points): How to let the application know which streaming I/O modes
>> are available? That's never been possible before, but it would be very nice
>> indeed if that's made explicit.
>
> Thanks for bringing up these considerations and questions, which perhaps
> cover the last missing bits for streamlined use of the request API by
> userspace. I would suggest another item, related to 3):
>
> 5) How can applications tell whether the driver supports a specific
> codec profile/level, not only for encoding but also for decoding? It's
> common for low-end embedded hardware to not support the most advanced
> profiles (e.g. H264 high profile).
>
>> Since the Request API associates data with frame buffers it makes sense to expose
>> this as a new capability field in struct v4l2_requestbuffers and struct v4l2_create_buffers.
>>
>> The first struct has 2 reserved fields, the second has 8, so it's not a problem to
>> take one for a capability field. Both structs also have a buffer type, so we know
>> if this is requested for a capture or output buffer type. The pixel format is known
>> in the driver, so HAS/REQUIRES_REQUESTS can be set based on that. I doubt we'll have
>> drivers where the request caps would actually depend on the pixel format, but it
>> theoretically possible. For both ioctls you can call them with count=0 at the start
>> of the application. REQBUFS has of course the side-effect of deleting all buffers,
>> but at the start of your application you don't have any yet. CREATE_BUFS has no
>> side-effects.
>
> My initial thoughts on this point were to have flags exposed in
> v4l2_capability, but now that you're saying it, it does make sense for
> the flag to be associated with a buffer rather than the global device.
>
> In addition, I've heard of cases (IIRC it was some Rockchip platforms)
> where the platform has both stateless and stateful VPUs (I think it was
> stateless up to H264 and stateful for H265). This would allow supporting
> these two hardware blocks under the same video device (if that makes
> sense anyway). And even if there's no immediate need, it's always good
> to have this level of granularity (with little drawbacks).
>
>> I propose adding these capabilities:
>>
>> #define V4L2_BUF_CAP_HAS_REQUESTS 0x00000001
>> #define V4L2_BUF_CAP_REQUIRES_REQUESTS 0x00000002
>> #define V4L2_BUF_CAP_HAS_MMAP 0x00000100
>> #define V4L2_BUF_CAP_HAS_USERPTR 0x00000200
>> #define V4L2_BUF_CAP_HAS_DMABUF 0x00000400
>>
>> If REQUIRES_REQUESTS is set, then HAS_REQUESTS is also set.
>>
>> At this time I think that REQUIRES_REQUESTS would only need to be set for the
>> output queue of stateless codecs.
>>
>> If capabilities is 0, then it's from an old kernel and all you know is that
>> requests are certainly not supported, and that MMAP is supported. Whether USERPTR
>> or DMABUF are supported isn't known in that case (just try it :-) ).
>
> Sounds good to me!
>
>> Strictly speaking we do not need these HAS_MMAP/USERPTR/DMABUF caps, but it is very
>> easy to add if we create a new capability field anyway, and it has always annoyed
>> the hell out of me that we didn't have a good way to let userspace know what
>> streaming I/O modes we support. And with vb2 it's easy to implement.
>
> I totally agree here, it would be very nice to take the occasion to
> expose to userspace what I/O modes are available. The current try-and-
> see approach works, but this feels much better indeed.
>
>> Regarding point 3: I think this should be documented next to the pixel format. I.e.
>> the MPEG-2 Slice format used by the stateless cedrus codec requires the request API
>> and that two MPEG-2 controls (slice params and quantization matrices) must be present
>> in each request.
>>
>> I am not sure a control flag (e.g. V4L2_CTRL_FLAG_REQUIRED_IN_REQ) is needed here.
>> It's really implied by the fact that you use a stateless codec. It doesn't help
>> generic applications like v4l2-ctl or qv4l2 either since in order to support
>> stateless codecs they will have to know about the details of these controls anyway.
>>
>> So I am inclined to say that it is not necessary to expose this information in
>> the API, but it has to be documented together with the pixel format documentation.
>
> I think this is affected by considerations about codec profile/level
> support. More specifically, some controls will only be required for
> supporting advanced codec profiles/levels, so they can only be
> explicitly marked with appropriate flags by the driver when the target
> profile/level is known. And I don't think it would be sane for userspace
> to explicitly set what profile/level it's aiming at. As a result, I
> don't think we can explicitly mark controls as required or optional.
>
> I also like the idea that it should instead be implicit and that the
> documentation should detail which specific stateless metadata controls
> are required for a given profile/level.
>
> As for controls validation, the approach followed in the Cedrus driver
> is to check that the most basic controls are filled and allow having
> missing controls for those that match advanced profiles.
>
> Since this approach feels somewhat generic enough to be applied to all
> stateless VPU drivers, maybe this should be made a helper in the
> framework?

Sounds reasonable. Not sure if it will be in the first version, but it is
easy to add later.

> In addition, I see a need for exposing the maximum profile/level that
> the driver supports for decoding. I would suggest reusing the already-
> existing dedicated controls used for encoding for this purpose. For
> decoders, they would be used to expose the (read-only) maximum
> profile/level that is supported by the hardware and keep using them as a
> settable value in a range (matching the level of support) for encoders.
>
> This is necessary for userspace to determine whether a given video can
> be decoded in hardware or not. Instead of half-way decoding the video
> (ending up in funky results), this would easily allow skipping hardware
> decoding and e.g. falling back on software decoding.

I think it might be better to expose this through new read-only bitmask
controls: i.e. a bitmask containing the supported profiles and levels.

Reusing the existing controls for a decoder is odd since there is not
really a concept of a 'current' value since you just want to report what
is supported. And I am not sure if all decoders can report the profile
or level that they detect.

Regards,

Hans